This invention relates to the joining of an X-ray tube anode assembly. More particularly, it relates to the joining of a shaft (or stud) for an X-ray anode target to the hub of a rotor member, using a high-temperature brazing alloy.
A principal component of conventional X-ray equipment and computed tomography (CT) equipment is an X-ray tube which provides the source of X-rays. Such tubes contain a vacuum at 10.sup.-8 to 10.sup.-9 torr and operate by accelerating a stream of electrons from a heated cathode through a high voltage against a high melting point target anode. Since only about 1% of the electron energy is converted by this method into X-rays and the remaining 99% converted into heat, considerable heat is generated in the anode as a by-product of the generation of X-rays.
In order to reduce heat concentration in the anode, the anode is rotated at speeds up to 10,000 rpm thereby continuously presenting the cathode with a new and cooler surface. In a high performance X-ray tube, the surface of the anode may reach temperatures of 3200.degree. C., and areas of the anode outside the immediate target surface may rise to temperatures of approximately 1300.degree. C.
Rotor hub assemblies in X-ray tube anodes are often composed of refractory metal alloys, such as molybdenum, niobium, titanium, zirconium, and nickel. These metals and others are present in different amounts in different components of the assemblies. For example, a hub component may have the alloy metals present in certain amounts, a stud or rotor shaft in other amounts, and a fastening nut in still other amounts. The purpose of using different alloys is to afford a high rate of heat dissipation as well as achieve performance requirements. The joining of these components for the rotor hub assemblies presents a problem of precisely matching the thermal properties of a brazing alloy which serves as a filler material with those of the high-temperature refractory metal alloys to be joined. The brazed joint must not only overcome any mismatch in thermal properties of the component parts, but must also obtain a mechanical and thermal integrity of the assembly. It will be appreciated that the anode target weighs up to 5 pounds and, as previously stated, rotates at 10,000 rpm. The operating temperature of the anode can be up to 1000.degree. C. at the stud/hub joint. In order to effect a reliable brazed joint under these conditions, the braze alloy must possess excellent high-temperature strength and ductility.
Brazing filler metals, which are commonly used in the industry for this type of application, include gold, silver and copper-based alloys. The problems with these filler materials under the conditions previously set forth are: Poor wetting and flow on some or all of the component materials being joined, leading to voids and other defects; low ductility, leading to cracking on cooling of the brazed joint, with very high stresses also created on the component piece parts, often leading to their cracking as well; formation of dispersed intermetallic brittle phases; and liquidus temperatures not much above 1000.degree. C., thus limiting the operating temperature of the assembly during service.